Traditional split/splitless (SSL) or programmed temperature vaporizing (PTV) injection ports for gas chromatographs typically consume large volumes of carrier gas by virtue of what is used at the split vent and septum purge vent rather than what is utilized for the actual analytical separation (column flow). To illustrate, a capillary column flow of approximately 1 standard cubic centimeter per minute (sccm) may have 50 sccm or more of split flow and 5 sccm of septum purge flow. One prior art method to reduce this consumption, e.g., “gas saver”, can reduce the split flow following an injection period. Reducing the split flow to too low a value, however, can result in undesirable elevated baselines. This may be caused by a continual outgassing of higher molecular weight contaminants introduced from the sample matrix, outgassing of polymeric seals such as O-rings, injection port septa and/or coring of such septa, or be caused by oxidation of the column stationary phase due to larger concentrations of oxygen which has back-diffused through the septum. Reducing these contaminants has traditionally been accomplished through dilution by using large split flows. Thus, significant quantities of carrier gas may be consumed, even during chromatograph idle periods.
FIG. 1 illustrates a conventional gas supply and exhaust system 25 and associated split/splitless injector 1 of the prior art. The split/splitless (SSL) injector 1 is provided for receiving injections of liquid samples from a syringe (not illustrated), for flash vaporizing the liquid samples by application of heat, for mixing the volatilized sample material with a carrier gas and for providing a portion of the volatilized sample material to a gas chromatograph column 20. The carrier gas supply, e.g. helium, is introduced under pressure into a gas inlet line 7 by means of a gas fitting 3. A controlled pressure of the carrier gas is provided to the injector by an electronic pressure controller (not shown) which results in a controlled flow of carrier gas through column 20. A fine porosity filter 4, e.g. a stainless steel fit, removes any particulate matter that may foul operation of a proportional valve 5 that is disposed downstream in the gas inlet line 7. The proportional valve 5 maintains a setpoint pressure within the body of the injector 1 in response to measurements provided by pressure sensor 8 in order to establish a calculated flow in the analytical column in accordance with the Poiseuille equation. The pressure sensor 8 provides a feedback loop to a control circuit of the electronic pressure controller. Optionally, a chemical trap 6 is included in the gas inlet line 7 to scrub the carrier gas of potential contaminants, e.g. hydrocarbons and/or oxygen.
As is known, generally only a portion (and, frequently, only a small portion) of the volatilized sample material and carrier gas flow actually enters the column 20. The remainder of the volatilized sample material and carrier gas flow is exhausted from the system by means of septum purge vent line 13 and, frequently, split flow vent line 14. Flow restrictor 11 and flow restrictor 18, disposed, respectively, in the septum purge vent line 13 and split flow vent line 14 maintain a pressure difference between the exhaust ports 12, 19, which are at ambient pressure, and the higher-pressure segments of the vent lines 13, 14 that are adjacent to the injector 1. Pressure sensors 10, 17 measure the pressures of the high-pressure segments of the vent lines 13, 14. This information is provided as continuous feedback to the electronic pressure controller which calculates the pressure differences across the flow restrictors 11, 18 and operates the proportional valves 9, 16 so as to maintain desired flow rates within the vent lines 13, 14, in accordance with a mathematical calculation or calibrated lookup table. A chemical trap 15 may be included in the split flow vent line 14 to protect the proportional valve 16 from contamination by various oils and greases that may volatilize from the sample or outgas from injector components. Another similar chemical trap (not shown) may also be included in the septum purge vent line 13.
The injector 1 includes two basic modes of operation: split and splitless. In the split injection mode, a split flow is established that exits the split vent line 14. This mode of operation is used for injection of concentrated analytes to prevent overloading of the column or saturation of the detection system used at the terminal end of the column. In the splitless mode of operation, the split vent line 14 is closed (i.e., proportional valve 16 is closed) during a sample injection to cause the bulk of the sample material to be transferred to the capillary column. After a specified time interval, the split vent, line is once again opened to vent residual solvent vapors and to dilute any contaminants that might outgas from contaminated surfaces.
In both the split and splitless modes, far greater amounts of carrier gas are used for split flow and septum purge flow than are required for the gas chromatography (GC) column flow carrying out the analytical separation. During idle times following a split or splitless injection, large volumes of split flow are typically maintained to dilute outgassing of residual contaminants. Even during idle times, when the chromatography system is not in use and the column is cooled to room temperature, the injector remains at above-ambient temperature and under a continued flow of gas. This results in a large consumption of high purity gas, e.g. helium.
Helium, which is a finite natural resource, is becoming increasingly expensive and difficult to procure in some areas of the world. Helium is often the preferred gas of choice due to sensitivity, efficiency, chemical inertness, safety or other concerns. Alternative carrier gasses, e.g. hydrogen or nitrogen, can be used in some instances. For a mass spectrometer detection based system, hydrogen decreases sensitivity for electron ionization (EI) and can cause dehydrohalogenation reactions in the ion source while nitrogen can result in charge exchange reactions, and is known to be less efficient as a carrier gas. Thus, there is a need in the art for gas chromatography systems that conserve helium by restricting the times at which helium gas flow is applied to only those times when such helium flow is required and by restricting the flow rate of helium at such times to be not significantly greater than the minimum flow rate required.